The present invention concerns a process for making a ceramic green body of a type which can be fired into a thin ceramic sheet or tape for use in a multilayer capacitor or as a conductor.
A multilayer capacitor is a stack of a number of ceramic capacitor sheets connected in parallel to produce a capacitor of high total capacitance. These capacitors may be very small, and typically comprise between 40 and 50 ceramic sheets, each about 20 .mu.m thick . The entire capacitor may be only about 1 mm thick. These devices fing many uses in microcircuitry.
However, there is a need for even thinner ceramic sheets or bodies, within the range of from about 3 .mu.m to about 10 .mu.m in thickness. This is due to the fact that equivalent capacitance could thereby be achieved using only half as many of these thinner sheets, since capacitance is inverity proportional to the thickness of the ceramic sheet. The reduction in the number of sheets would then result in a savings in material costs, because the electrode layers interspersed between the ceramic layers in a capacitor are made of expensive noble metals, and with fewer, thinner sheets, fewer electrode layers would be required. In past it has been very dificult to make sheets of this reduced thickness because current commercial methods of producing the green (unfired) sheets, also called tapes, involve processing of solides, including ceramic powders.
These ceramic powders, which are dielectric and/ar conductive, are generally dispersed into an oranic solvent such as methyl ethyl ketone/ethanol mixture. The powders are often barium titanate admixed with other compounds such as strontium titanate, lead titanate, calcium zirconate, lead oxide, borates and silicates. Ball-milling is generally necessary to maximize dispersion in the solvent, and often requires several hours at a minimum. The dispersed powders are then mixed with polymeric organic binders, plasticizers and surfactants to form a slip, which is tape-cast onto a nonporous substrate and dried in an oven to form a flexible "green tape."
To produce a capacitor, this green tape or green body is screen-printed with a noble metal electrode ink. Forty or Fifty layers, typically, of the printed tape are then laminated and, after dicing into chips or sheets, fired to burn off the organic binder. The ingoranic chips are sintered at high temperatures, typically between 1000.degree. C. and 1400.degree.0 C., to densify the sheets and improve their strength and conductivity. The result is a mechanically and electrically acceptable device.
A problem very commonly faced in this process, however, that tends to militate against reducing the thicknell of the sheets or tapes beyond thickness currently being produced, is the fact that ceramic powders are usually agglomerated when received and remain so to some extent even after lengthy ball-milling prior to and after dispersion in a solvent. This agglomeration makes it difficult to produce sheets of only from about 3 .mu.m to about 10 .mu.m in thickness that are of uniform quality, since the size and shape of the initial ceramic particles are critical factors in producing a good quality final product. This process also tends to produce sheets that exhibit loading levels that are less than optimal, and the reduced density increases the degree of shrinkage of the tape during firing. Ideally, the particles should be uniformily sized and equiaxially shaped, and should also demonstrate high purity.
Therefore, because of the problems associated with the ceramic powders and their general unsuitability to producing the thinner ceramic sheets or tapes now sought, it would be desirable to have a cermaic green body prepared from a slip fromulation which does not generally require ball-milling or other milling steps at any point and for which powder agglomeration does not present a significant problem, which exhibits good ceramic loading levels, and which therefore does not generally require processing of dry ceramic powders, with the accompanying quality problems and thickness limitations that such processing entails.